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The problem of the external world is a classic challenge in epistemology (the theory of knowledge). This challenge, which was first presented by the ancient skeptics, is met by proving that what I seem to be experiencing is actually real. As an example, it would require proving that the computer I seem to be typing this on exists outside of my mind.
Some of the early skeptics generated the problem by noting that what seems real could be just a dream, generated in the mind of the dreamer. Descartes added a new element to the problem by considering that an evil demon might be causing him to have experiences of a world that does not actually exist outside of his mind. While the evil demon was said to be devoted to deception, little is said about its motive in this matter. After Descartes there was a move from supernatural to technological deceivers: the classic brain-in-a-vat scenarios that are precursors to the more recent notion of virtual reality. In these. . .